Systems such as IEEE 802.11 (WiFi), IEEE802.16 (WiMAX), 3.9G wireless access network communication (LTE) and 4G wireless access network communication (4G) are listed as a communication system employing frequency division multiplex transmission such as OFDM transmission including OFDMA.
A transmission system using OFDM uses RF frequencies in a broad range by means of a multiple carrier system. Further, the transmission system using OFDM can have robustness against frequency selective fading by using error correction technology and so on. In some cases, a system using OFDM transmission changes a modulation method of a subcarrier forming multiple carriers.
Meanwhile, an antenna to be used for a transmission device configured to perform OFDM transmission does not have a standing wave ratio characteristic (SWR characteristic) which is uniform in a whole RF frequency range. An ordinary antenna generally has a frequency-SWR characteristic illustrated in FIG. 1. In FIG. 1, the SWR is small and impedance is matched at a central frequency. Meanwhile, in FIG. 1, the SWR grows and impedance matching becomes insufficient as the frequency goes away from the central frequency. There is a thought of giving an antenna a plurality of resonant frequencies as illustrated in FIG. 2. If the SWR characteristic is different from the whole RF frequency range in which the OFDM transmission is performed as illustrated in FIG. 1 or 2, the SWR characteristic is affected by a frequency-gain deviation. Thus, there is a problem that the OFDM signal transmission system of a broad frequency range is affected by the frequency-gain deviations of the antenna and feeder systems illustrated in FIG. 1 and FIG. 2. A significant effect of the frequency-gain deviation may cause a problem that an opposite receiver produces lots of errors of signal identification.
Japanese Laid-Open Patent Publication No. 2002-237795 is known as an example of related art.